The invention concerns a process for producing a tube-shaped target for cathode sputtering plants, in which the tube-shaped target is formed from a metallic inner tube (target holder) made of a first material with a first melting point of Ts1xe2x89xa7900 K and a metallic outer tube (target) that concentrically surrounds the inner tube and that is made of a second material with a second melting point of Ts2xe2x89xa6800 K. The inside diameter of the outer tube and the outside diameter of the inner tube are proportioned in such a way that the two tubes fit together tightly and are mechanically firmly joined. The invention also concerns the use of the process. Tube-shaped targets or rotating targets are increasingly preferred over planar targets for producing thin coatings, since they allow higher sputtering yields in the physical vapor deposition process or sputtering process. The use of tube-shaped targets to produce thin oxide coatings by reactive sputtering of metals under oxygen-containing atmospheres is especially advantageous. In this case, the oxides of low-melting metals, such as tin, zinc, indium, bismuth, or their alloys, are preferably deposited as thin coatings. These low-melting metals are subject to creep even at room temperature or at the slightly elevated temperatures that prevail during the sputtering process. Creep occurs at temperatures that are equal to or greater than 40% of the melting point of the metal. To prevent creep deformation, outer tubes made of these types of metals are usually supported by an inner tube that consists of a material with a higher melting point and that is usually cooled. It is necessary to produce contact with good adhesion between the two tubes over their entire surface area in order to ensure good heat transfer.
U.S. Pat. No. 5,354,446 describes various production processes for tube-shaped targets with an inner tube or target support tube and an outer tube or target tube made of soft, low-melting or fracture-susceptible metals or alloys. In one of these processes, the outer tube is applied to the inner tube by thermal spraying. In another process, the tube outer tube is joined to the inner tube with indium solder. Furthermore, the cited patent describes the use of adhesion-improving coatings between the inner and outer tubes, which allow adaptation of the different coefficients of thermal expansion of the inner and outer tubes. In addition, a process is described in which the outer tube is applied to the inner tube by hot isostatic pressing.
The cited processes are expensive, labor-intensive, and problematic. For example, the full-surface joining of the inner and outer tubes by soldering is generally difficult due to the geometric circumstances. This process turns out to be especially difficult when an outer tube made of a low-melting material is to be soldered, since the melting points of the outer tube and the solder often fall within very similar ranges.
Application of the outer tube by thermal spraying causes irregularities in the structure of the outer tube. On the one hand, these irregularities are caused by gas inclusions in the form of pores, gas inclusions in dissolved form, or inclusions of oxide particles in sprayed metallic coatings. On the other hand, the necessity of applying the outer tube in layers due to the process technology involved in thermal spraying results in a nonuniform structure of shells built up in the outer tube, which results in poor adhesion to the inner tube. These types of structural problems manifest themselves in nonuniform deposition rates during the cathode sputtering process. Gas inclusions in pores can lead to deflagration and material spalling, since the sputtering is carried out in a vacuum. Oxygen dissolved in the structure affects the stoichiometry and makes it difficult to control during the deposition of an oxide.
The production of cylindrical metal tubes by casting is also well known. For example, DE 2 427 098, DE 3 532 131 A1, and DE 4 216 870 A1 each discloses a casting process for producing metal parts with permanent molds, in which the goal is to achieve directional solidification of the molten metal for the purpose of producing a directional structure that is as uniform as possible. The directional solidification of the molten metal is achieved by cooling in a directional temperature field, such that the permanent mold is cooled from the bottom to the top. The thermal gradient is achieved, for example, by lowering the mold into an immersion bath, cooling it with air, or surrounding it with heating elements.
EP 092 477 describes the vertical core casting of a steel ingot in a permanent mold, in which a cooled, hollow metal mandrel is used as the core. A gas or vapor is fed into the hollow mandrel for cooling, such that the gas or vapor flows from the bottom of the mold through the hollow mandrel towards the top of the mold. The hollow mandrel adheres to the steel ingot after the mold has cooled, but no metal joining occurs between the two, as would occur in welding or soldering. This prevents a breakthrough or melt-through of the hollow mandrel during casting and the formation of hot cracks in the cast steel ingot.
Accordingly, it is an object of the present invention to provide a simple and inexpensive process for producing tube-shaped targets for cathode sputtering plants, with which a high degree of purity and a directional structure can be produced in an outer tube with a melting point of xe2x89xa6800 K to be used as a sputtering target. There is the further object of providing an application for a process of this type.
In accordance with the invention, this object is achieved by forming the outer tube by casting the second material in a molten state in a heated, vertical, cylindrical permanent mold, which has a heated mandrel that constitutes the inner tube, in such a way that, after a space between the mold and the inner tube has been filled with the molten second material, a first thermal gradient develops between the inner tube and the mold. A second thermal gradient develops between the bottom and the top of the mold, and the outer tube is simultaneously cooled from the inside to the outside and from the bottom to the top.
This process has the advantage that a sufficient amount of metal feed can continue to be supplied to the casting, so that the formation of blowholes is minimized. Entrapped gas bubbles are able to move upward and escape from the mold. A directional, very uniform, and clean structure with columnar crystallites is formed, with the columns aligned perpendicularly to the surface of the outer tube. In addition, a metallic bond is produced between the outer and inner tube, which results in a high level of adhesion and ideal heat transfer.
The second material is preferably melted under an atmosphere of inert gas or in a vacuum and then fed into the mold by a molten metal pump or a siphon to suppress the formation of oxides and impurities as much as possible.
The second thermal gradient is preferably produced by heating the mold with at least two separately controllable heating devices that radially surround the mold. In this connection, it is advantageous to use electric heating mats as the heating devices. Heating mats of this type are made of heating conductors embedded in an insulating material, and their flexibility makes it possible to fit them snugly around the mold. The inner tube is preferably heated with hot gas or steam at its inside diameter.
The mold and inner tube are preferably preheated to a temperature of 400-850 K before the casting process begins. Preheating prevents solidification of the melt from occurring too quickly on the inside wall of the mold and on the outside wall of the inner tube, which would lead to irregularities in the structure of the outer tube. It was found to be especially advantageous to preheat at least the mold to a temperature that is greater than the melting point Ts2 of the second material, so that solidification of the molten metal on the inside wall of the mold is completely prevented during the filling of the mold.
It has been found to be advantageous to use high-grade steel, aluminum, or copper as the first material. The melting points of these materials are significantly higher than those of the second material, which completely rules out the possibility that the inner tube could melt through during the casting process.
Tin, zinc, bismuth, indium, or an alloy of these metals is preferably used as the second material.
To improve the formation of the metallic bond between the outer tube and the inner tube, the inner tube can be pretreated before the outer tube is cast by applying a bonding agent to the outside surface of the inner tube in a coating thickness of 5-500 xcexcm. For this purpose, a coating of solder can be applied as the coating of bonding agent, with the use of a flux. The coating of bonding agent can be produced at least in part by electro-deposition or electroless plating, or it can be produced by thermal spraying.
It has been found to be effective to use an inner tube made of high-grade steel and to apply the coating of solder as the bonding agent with the use of the flux, such that the coating of solder consists at least partly of the second material. It has also been found to be effective to use an inner tube made of aluminum or copper and to produce the coating of bonding agent at least in part by electro-deposition or electroless plating, such that the coating of bonding agent consists at least partly of the second material. After electro-deposition or electroless plating, it may make sense to additionally apply a coating of solder. It has also been found to be effective to produce the coating of bonding agent by thermal spraying, especially the thermal spraying of nickel.
The first thermal gradient is ideally produced by discontinuing the heating of the inside diameter of the inner tube. In addition, it has been found to be effective to pass cold gas or water vapor through the inside of the inner tube. The second thermal gradient is preferably produced by shutting off the two or more heating devices at different times. The heating device near the bottom of the mold is shut off first, and then the one or more remaining heating devices adjacent to and above the first heating device are shut off after a certain amount of delay. In this connection, one separately controllable heating device should be provided for each 30-50 cm of vertical length of the mold in order to produce a smooth course of the second thermal gradient.
To allow easy removal of the tube-shaped target from the mold after it has cooled, the mold should consist of at least two radially separable parts.
The process of the invention is preferably used to produce outer tubes with a thickness of 3-12 mm.
Ideally, the process is used to produce a tube-shaped target for cathode sputtering plants with an oxygen concentration in the outer tube of xe2x89xa6100 ppm and a structure composed of columnar crystallites that are aligned perpendicularly to the surface of the outer tube.
The process is preferably used to establish an oxygen concentration of xe2x89xa650 ppm and an average particle size of the columnar crystallites of 0.3 mm to 15 mm.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference is made to the drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.